Apparatus for memory device testing and field applications

ABSTRACT

A memory test system is disclosed that includes a memory integrated circuit (IC) and a memory functional tester. The memory IC includes a plurality of memory banks, where each memory bank includes a plurality of memory cells. The memory functional tester includes an adjustable voltage generator circuit, a read current measurement circuit, and a controller. The memory functional tester performs a write/read functional test on the memory bank over a number of write control voltages to determine a preferred write control voltage, where the preferred write control voltage is designated for use during subsequent write operations to the memory bank during an operational mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 15/993,709, filed May 31, 2018, which claims the benefit ofU.S. Provisional Patent Appl. No. 62/527,901, filed Jun. 30, 2017, bothof which are incorporated herein by reference in their entirety.

BACKGROUND

A data storage device is an electronic device for writing and/or readingelectronic data. The data storage device can be implemented as volatilememory, such as random-access memory (RAM), which conventionally requirepower to maintain its stored information or non-volatile memory, such asread-only memory (ROM), which can maintain its stored information evenwhen not powered. RAM can be implemented in a dynamic random-accessmemory (DRAM), a static random-access memory (SRAM), and/or anon-volatile random-access memory (MRAM), often referred to as a flashmemory, configuration. The electronic data can be written into and/orread from an array of memory cells which can be accessible throughvarious control lines.

Magnetic Random Access Memory (MRAM) and Resistive Random Access Memory(RRAM) are two types of recently developed memory devices. MRAM and RRAMare applicable for embedded memory, DRAM replacement, flash replacement,among other applications. MRAM and RRAM devices are inherently sensitiveto process variations during device fabrication. Therefore, memoryperformance variations during testing are observed from die-to-dieacross a semiconductor wafer, and even block-to-block within a singleintegrated circuit (IC) die. Different die at different wafer locations(e.g. center die vs. edge die) and different blocks of a single largeMRAM OR RRAM IC can often have very different read/write windows, andfail the performance test of read/write window margin at a very highrate, which can limit the usefulness of the MRAM and the RRAM devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a block diagram of a memory test system according toan exemplary embodiment of the present disclosure.

FIG. 2 illustrates a number of read current distributions that resultfrom performing a write/read functional test over the range of availablewrite control voltages according to an exemplary embodiment of thepresent disclosure.

FIG. 3A illustrates a flowchart of an exemplary operation for performinga functional test of a memory device, according to an exemplaryembodiment of the present disclosure.

FIG. 3B illustrates a flowchart of a second exemplary operation forperforming a functional test of a memory device according to anexemplary embodiment of the present disclosure.

FIG. 4 illustrates a block diagram of memory device accordingly to asecond exemplary embodiment of the disclosure.

FIGS. 5A and 5B illustrate circuit diagrams of an adjustable voltagegenerator circuit according to exemplary embodiments of the presentdisclosure.

FIGS. 6A and 6B illustrate circuit diagrams of anti-fuse circuits thatcan be used in a memory device according to exemplary embodiments of thepresent disclosure.

FIG. 7 illustrates a circuit diagram of a fuse circuit that can be usedin a memory device in exemplary embodiments of the present disclosure.

FIG. 8 illustrates an MRAM cell according to embodiments of the presentdisclosure.

FIG. 9 illustrates an RRAM cell according to embodiments of the presentdisclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over a second feature in the description that followsmay include embodiments in which the first and second features areformed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is does not in itselfdictate a relationship between the various embodiments and/orconfigurations discussed.

Overview

A memory test system is provided that includes a memory integratedcircuit (IC) and a memory functional tester. The memory IC includes anumber of memory banks, where each memory bank includes a number ofmemory cells. The memory functional tester performs a write/readfunctional test on the memory bank over a number of write controlvoltages to determine a preferred write control voltage. The memoryfunctional tester includes an adjustable voltage generator circuit, aread current measurement circuit, and a controller. The adjustablevoltage generator circuit generates the number of write control voltagesto flip a logic state stored in the number of memory cells duringcorresponding write cycles of the write/read functional test. Theadjustable voltage generator circuit further generates a number of readcontrol voltages for the number of memory cells during correspondingread cycles of the write/read functional test. The read currentmeasurement circuit measures a number of read currents generated by thenumber of memory cells that are received responsive to the number ofread control voltages during the corresponding read cycles of thewrite/read functional test. The controller determines a number of errorrates based on the number of read currents, where each error rate isassociated with a corresponding write control voltage of the number ofthe write control voltages, and selects a preferred write controlvoltage from the number of write control voltages based on a comparisonof the number of error rates with each other. The preferred writecontrol voltage is designated for use during subsequent write operationsto the memory bank during an operational mode.

Exemplary Memory Test System

FIG. 1 illustrates a memory test system 100 according to an exemplaryembodiment of the present disclosure. Memory test system 100 includesmemory integrated circuit (IC) 102 and a functional tester 104, wherememory IC 102 can be a MRAM or RRAM memory device and functional tester104 can and MRAM/RRAM functional tester.

Memory IC 102 is divided into memory banks 106 a-d, where each memorybank 106 a-d includes a number of individual memory cells. Memory IC 102further includes a controller/interface 110 to provide an interfacebetween memory IC 102 and the functional tester 104 and other externalcircuits. The memory banks 106 a-d can be geographically dispersedacross the memory IC 102, and therefore subject to disparate processvariations from one memory bank 106 a-d to another, which can affectmemory performance if not addressed. Further, each memory bank 106 a-dhas an associated fuse/antifuse circuit 108 that implements a preferredwrite control voltage and/or read control voltage for the correspondingmemory bank 106, where the preferred write control voltage and/or readcontrol voltage is determined during functional testing as will bedescribed below. Memory banks 106 a-d are illustrated as four memorybanks, but any number of memory banks can be used as will be understoodby those skilled in the relevant art.

As will be described, functional tester 104 performs a write/readfunctional test on each memory bank 106 a-d, in order to test the memoryIC 102 over a range of write control voltages that are stepped andrepeated. For a given write control voltage in the range of writecontrol voltages, the functional test is performed by writing a firstlogic state (e.g.,“1”) into the memory cells of a memory bank 106 a-dunder test, and then subsequently reading back the logic states storedin memory cells of the memory bank 106 a-d under test and recording theread current that is measured for each memory cell. Given the number ofmemory cells in the memory bank under test and their variation insemiconductor performance, this will result in a first distribution ofread currents for the memory bank 106 a-d under test that represent thefirst logic state. The process is repeated for the second logic stage(e.g. “0”) for the given write control voltage, resulting in a seconddistribution of read currents for the memory bank 106 a-d under testthat represent the second logic state. An error rate is then determinedfor the memory bank under test for the particular write control voltage,where the error rate is determined based on the number of read currentsthat are found to be indeterminate, e.g., those read currents thatcannot be classified as either logic “1” or a logic “0” because of theirmagnitude. The functional test is repeated for each write controlvoltage over the range of write control voltages, resulting in acorresponding n-number of error rates. A preferred write control voltageis then selected from the range write control voltages for operationaluse based on the error rates. For example, in an embodiment, the writecontrol voltage having the lowest error rate can be selected as thepreferred write control voltage. Afterwhich, read detection windows aredetermined based on the first and second read currents distributionsassociated with the preferred write control voltage.

Functional tester 104 includes an adjustable voltage generator circuit112, a read current measurement circuit 114, and a controller 116, wherethe controller 116 controls the operation of the adjustable voltagegenerator circuit 112 and the current measurement circuit 114 viacontrol signals, such as control signals 126 and 124, respectively.Controller 116 can include one or more processors operating according toinstructions, such as computer instructions that are stored in aninternal memory or an external memory, as will be understood by thoseskilled in the relevant art.

The functional test of each of the memory banks 106 a-d includes a firstwrite/read cycle followed a second write/read cycle to capturecorresponding read current distributions for the two possible logicstates (e.g., “0” and “1”) that can be stored in the memory cell. Duringthe first write cycle, the controller 116 directs the adjustable voltagegenerator circuit 112 to generate a (first) write control voltage 118that is provided to the control interface 110 of the memory IC 102. The(first) write control voltage 118 is a one of n-number of write controlvoltages 118 that will be tested over a predetermined range. Thecontroller interface 110 of the memory IC 102 applies the (first) writecontrol voltage 118 to the memory cells of a particular memory bank 106a-d that is under test, such as memory bank 106 a for purposes ofdiscussion. Generally, a relatively high voltage (e.g. 2.5 volts) isrequired to “flip” the logic state of an MRAM memory cell or a RRAMmemory cell (e.g. flip from a “0” to a “1”, or vice versa). Whereas, arelatively low voltage will not “flip” the logic state of the MRAMmemory cell or RRAM memory cell, and therefore the stored logic stateremains the same for the relatively low voltage. Accordingly, assumingthe initial logic state stored in the memory cells in memory bank 106 aare a logic “0”, the adjustable voltage generator circuit 112 generatesa relatively high write control voltage 118 (e.g., 2.5 volts) to beapplied to the memory cells in the memory bank 106 a, in order to flipthe logic states from a logic “0” to a logic “1”.

During the first read cycle of the functional test, the controller 116directs the adjustable voltage generator circuit 112 to generate a readcontrol voltage 120 that is provided to the control interface 110 of thememory IC 102. The controller interface 110 of the memory IC 102 appliesthe read control voltage 120 to the memory cells of the memory bank 106a. The read control voltage 120 generally has a lower voltage magnitudethan the write control voltage 118, so that the logic state stored inthe memory cells of the memory bank 106 a are not flipped (again) duringthe read cycle by the read control voltage 120. For example, the readcontrol voltage 120 can be a fraction (e.g. 50%) of the write controlvoltage 118. The read control voltage 120 causes each memory cell of thememory bank 106 a to generate a read current 122 that is forwarded tothe functional tester 104 by the control interface 110 for evaluation.The read current measurement circuit 114 in the functional tester 104measures the read current 122, which indicates a stored logic state ofthe particular memory cell of the memory bank 106 a. Generally, arelatively low read current 122, indicates that a memory cell of thememory bank 106 a is in a high resistance state, which represents alogic “0” being stored in the memory cell. Whereas, a relatively highread current 122, indicates that the memory cell of the memory bank 106a is in low resistance state, which represents a logic “1” being storedin the memory cell of the memory bank 106 a. The read currentmeasurement circuit 114 provides a current measurement 124 for eachmemory cell in the memory bank 106 a to the controller 116. Given thenumber of memory cells in the memory bank 106 a under test, this resultsin a first distribution of read currents that represent a logic “1”,e.g. low resistance state assuming the memory bank 106 a initiallystored logic “0” and was flipped to a logic “1” by the (first) writecontrol voltage 118.

The second write/read cycle is substantially a duplicate of the firstwrite/read cycle of the functional test, so as to capture a seconddistribution read currents that represent a logic “0”, e.g. the highresistance state assuming the memory bank 106 a previously stored alogic “1.” More specifically, after the first write/read cycle, the(first) write control voltage 118 is applied again to the memory cellsof a memory bank 106 a so as to “flip” the logic state of an MRAM memorycell or a RRAM memory cell (e.g. flip from a “1” to a “0”) to measurethe high resistance state. The read voltage 120 is again applied to thememory cells of the memory bank 106 a, resulting in a seconddistribution of read currents 122 that represent a logic “0”, e.g. highresistance state of the memory bank 106 a. Accordingly, the controller116 now has first and second read current distributions that representthe collective read currents 122 for the corresponding low resistance(e.g. logic “1”) and high resistance (e.g. logic “0”) logic states ofthe memory bank 106 a under test when the (first) write control voltage118 is used to “flip” the logic state stored in the memory cells.

The functional test described above is repeated for each write controlvoltage 118 in the range of write control voltages that are availablefor use, resulting in n-number high resistance and low resistancecurrent distributions. For example, the write control voltage 118 can bestepped in predetermined voltage increments, (e.g., 0.1v) over the rangeof available write control voltages, resulting in n-number of writecontrol voltages 118 and the corresponding write/read functional testsbeing performed as described above.

FIG. 2 illustrates the current distributions 202-1 through 202-n thatresult from performing the write/read functional test over the range ofavailable write control voltages 118-1 through 118-n for an examplememory bank 106 a. As can be seen, current distributions 202-1 through202-n include respective low resistance current distributions 204-1through 204-n and respective high resistance current distributions 206-1through 206-n that correspond to their respective logic states “1” and“0”. The low resistance current distributions 204-1 through 204-n aregenerally greater magnitude than their respective high resistancecurrent distributions 206-1 through 206-n, for each respective writecontrol voltage 118-1 through 218-n. Further, the respective currentdistributions 202-1 through 202-n have differing “spreads” indistribution currents for the differing write control voltages 118-1through 218-n. For example, current distribution 202-1 (resulting fromwrite control voltage 118-1) has a wider “spread” than that of currentdistribution 202-3, because the individual currents are bunched tightertogether for current distribution 202-3. As indicated above, therelative performance of the memory bank 106 a under test will vary overprocess corners, die location, and also write control voltages 118 asillustrated by FIG. 2. As will be understood by those skilled in theart, it is generally more favorable to have a narrow “spread” for thecurrent distributions 204-1 through 204-n and 206-1 through 206-n, asthis generally leads to greater separation between the low resistancecurrent distributions 204-1 through 204-n and their respective highresistance current distribution 206-1 through 206-n, so that readcurrents 122 can be easily assigned to a logic state during theoperational mode.

Further, current distributions 202-1 through 202-n include a number ofrespective error currents, collectively represented as error currents208-1 through 208-n in respective current distributions 202-1 through202-n. Each of these error currents 208-1 through 208-n has a magnitudethat falls in between the respective low resistance currentdistributions 204-1 through 204-n and the respective high resistancecurrent distributions 206-1 through 206-n, as shown in FIG. 2.Accordingly, the respective logic states represented by error currents208-1 through 208-n are indeterminate, and therefore define the errorrate for the memory bank 106 a at the corresponding write controlvoltage 118. The various error currents 208-1 through 208-n for thecorresponding write control voltages 118-1 through 218-n can be analyzedand compared to select the write control voltage that produces thelowest error rate for the memory bank 106 a under test. For example, inFIG. 2, the current distribution 202-3 generated using the write controlvoltage 118-3 produces the least number of error currents 208-3 duringthe functional test of the memory bank 106 a. Accordingly, the writecontrol voltage 202-3 can be selected as the preferred write controlvoltage 118 that is to be used for the memory bank 106 a whensubsequently used for data storage during an operational mode, becausethe write control voltage 202-3 provides the lowest error rate of therange of available write control voltages 118. Likewise, the preferredread control voltage 120 can be determined as a fraction (e.g. 50%) ofthe preferred write control voltage 118.

The error rate associated with each of the write control voltages 118-1through 218-n is determined as the number of error currents determinedfor the particular write control voltage 118 divided by the total numberof memory cells in the memory bank 106 a, which is also the same as thenumber of read currents 122 generated during a given read cycle of thefunctional test. The controller 116 can then determine if the error rateexceeds a predetermined threshold, and if so then the memory bank 106 ais marked “fail” and is not used for field applications during theoperational mode. Otherwise, it is marked a “pass” so that it can beused to store data during the operational mode.

Once the preferred write control voltage 118 is selected and memory bank106 a is determined to have passed the error rate test, the controller116 can determine first and second read detection windows that can beused to determine first and second logic states for the preferred writecontrol voltage 118 during the operational mode. Referring to FIG. 2,read detection window 210 is determined for the logic state “1” and theread detection window 212 is determined for the logic state “0” giventhe current distributions 202-3 provided by the preferred write controlvoltage 118-3. Specifically, the read detection window 210 is determinedto fit the low resistance current distribution 204-3, and the readdetection window 212 is determined to fit the high resistance currentdistribution 206-3 determined during the functional test. Accordingly,during operational use of the memory bank 106 a, future read currents122 will be compared against the read detection windows 210 and 212 todetermine whether they represent a logic state “1” or a logic state “0”being stored in the corresponding memory cells of the memory bank 106 a.As a result of this procedure, it is noted that the first and secondread detection windows 210 and 212 are selected for the memory bank 106a based on the current distributions provided by the preferred writecontrol voltage 118 that was determined for the memory bank 106 a.

The functional test procedure described above is repeated for eachmemory bank of the number of memory banks 106 a-d. As a result, apreferred write control voltage 118 and a preferred read voltage 120 aredetermined for each of the memory banks 106 a-d, assuming that thefunctional test is passed for the corresponding memory bank 106. Afterwhich, the preferred write control voltage 118 and preferred readcontrol voltage 120 are permanently “locked-in” by the correspondingfuse/anti-fuse circuit 108 for each memory bank 106, so that thepreferred write control voltage 118 and read control voltage 120 aresubsequently used for field applications of the corresponding memorybank 106. Likewise, the first and second read detection windows 210 and212 can also be stored in a memory for subsequent use during future readoperations for read current evaluation. Herein, a fuse/anti-fuse circuit108 can include a fuse circuit and/or an anti-fuse circuit to performthe described functionality, as will be understood by those skilled inthe art.

FIG. 3A illustrates a flowchart 300 of an exemplary operation forperforming manufacturing including a functional test of a memory device,such as an MRAM or RRAM device, according to an exemplary embodiment ofthe present disclosure. The flowchart 300 makes reference to the memorytest system 100 in FIG. 1 and the read current distributions 202 in FIG.2 for example purposes only. The disclosure is not limited to thisoperational description or its application to memory test system 100.Rather, it will be apparent to ordinary persons skilled in the relevantart(s) that other operational control flows, systems, and applicationsare within the scope and spirit of the present disclosure.

At step 301, a memory device, such as memory IC 102 having memory banks106 a-d, is fabricated (e.g. manufactured) using techniques that areknown to one skilled in the art. The known semiconductor fabricationtechniques can include one or more of: deposition, planarization,lithography, diffusion or ion implantation, and/or other semiconductorwafer processing techniques that are known to one skilled in the art.

At step 302, a memory bank is selected from a number of memory banks 106a-d for a write/read functional test. For example, memory bank 106 a canbe selected for the functional test for discussion purposes.

At step 304, the functional test is performed on the selected memorybank 106. For example, the functional tester 104 can perform thewrite/read functional test, as described above, applying a range ofwrite control voltages 118 and corresponding read control voltages 120to the memory bank 106 a. The outcome of the functional test providesthe preferred write control voltage 118 that the memory bank 106 a canuse during the operational mode and the corresponding error rateassociated with the preferred write control voltage 118.

At step 306, it is determined whether the memory bank 106 a-d under testpassed the functional test based on the error rate associated with thepreferred write control voltage 118. For example, the controller 116 candetermine whether the memory bank 106 a passes the functional test bydetermining whether the error rate associated with preferred writecontrol voltage 118 exceeds a predetermined threshold. If error rateexceeds the predetermined threshold, then the memory bank 106 a failsthe functional test, and then control flows to step 302 to selectanother memory bank 106 a-d for test. If the error rate is below thepredetermined threshold, then the memory bank 106 a passes thefunctional test, and control flows to step 308.

At step 308, once the memory bank 106 a-d under test is determined tohave passed the error rate test, the first and second read detectionwindows can be determined for the memory bank 106 a-d under test. Forexample, the controller 116 can determine first and second readdetection windows associated the preferred write control voltage 118,and that can be used to determine first and second logic states duringthe operational mode of the memory bank 106 a under test. Referring toFIG. 2, read detection window 210 is determined for the logic state “1”and the read detection window 212 is determined for the logic state “0”given the current distributions 202-3 associated with the preferredwrite control voltage 118-3. Specifically, the read detection window 210is determined to fit the low resistance current distribution 204-3, andthe read detection window 212 is determined to fit the high resistancecurrent distribution 206-3 determined during the functional test.Accordingly, during operational use of the memory bank 106 a, futureread currents 122 will be compared against the read detection windows210 and 212 to determine whether the read currents represent a logicstate “1” or a logic state “0” being stored in the corresponding memorycells. If a future read current 122 falls outside both the readdetection window 210 and the read detection window 212, then theassociated logic state is indeterminate.

At step 310, the preferred write control voltage 118 and the preferredread control voltage 120 for the memory bank 106 a-d under test are“locked-in” for future use by the memory bank 106 a-d under test. Forexample, the fuse/anti-fuse circuit 108 for the memory bank 106 a can bemanipulated so that the preferred write control voltage 118 and thepreferred read control voltage 120 are used by the memory bank 106 a forfuture write/read applications, including field use application duringthe operational mode. Likewise, the first and second read detectionwindows 210 and 212 can also be stored in a memory for subsequent useduring future read operations for read current evaluation.

FIG. 3B illustrates a flowchart 350 of an exemplary operation forperforming a functional test of a memory bank, such as an MRAM or RRAMmemory bank, according to an exemplary embodiment of the presentdisclosure. More specifically, flowchart 350 further describes step 304in flowchart 300 of FIG. 3A, as applied to an exemplary memory bank 106a-d under test, such as memory bank 106 a.

At step 352, during the first write cycle, a current write controlvoltage 118 is applied to individual memory cells of the memory bank 106a to “flip” the logic state stored in the individual memory cells of thememory bank 106 a. For example, the adjustable voltage generator circuit112 can generate a write control voltage 118 that is provided to thecontrol interface 110 of the memory IC 102. The controller interface 110of the memory IC 102 applies the write control voltage 118 to the memorycells of the memory bank 106 a. Assuming that the initial logic statewas “0”, then the individual memory cells can receive the same writecontrol voltage 118 to flip the logic states from “0” to an expectedlogic state of “1”.

At step 354, during the first read cycle of the functional test, acurrent read control voltage 120 is applied to individual memory cellsof the memory bank 106 a to enable reading the actual logic statesstored in the memory cells of the memory bank 106 a. For example, theadjustable voltage generator circuit 112 can generate a read controlvoltage 120 that is provided to the control interface 110 of the memoryIC 102. In an embodiment, the current read control voltage is set to afraction (e.g. 50%) of the current write control voltage 118. Thecontroller interface 110 of the memory IC 102 applies the read controlvoltage 120 to the memory cells of the memory bank 106 a. The readcontrol voltage 120 causes the memory cells of the memory bank 106 a toeach generate a first read current 122 that is forwarded to thefunctional tester 104 by the control interface 110 for evaluation.

In step 356, the read current is measured for each memory cell of thememory bank 106 a. For example, the read current measurement circuit 114in the functional tester 104 measures the read current 122 to generate aread current measurement 124 for each memory cell in the memory bank 106a. Generally, a relatively low read current 122, indicates that a memorycell of the memory bank 106 a is in a high resistance state, whichrepresents a logic “0” being stored in the memory cell. Whereas, arelatively high read current 122, indicates that the memory cell of thememory bank 106 a is in low resistance state, which represents a logic“1” being stored in the memory cell of the memory bank 106 a. The readcurrent measurement circuit 114 provides a current measurement 124 foreach memory cell in the memory bank 106 a to the controller 116,resulting in a first set of read current measurements 124 that arereceived for evaluation. Given the number of memory cells in the memorybank 106 a-d under test, this results in a first distribution of readcurrents that represent a logic “1”, e.g. low resistance state assumingthe memory bank 106 a initially stored logic “0” and was flipped to alogic “1” by the current write control voltage 118.

At step 358, during the second write cycle, the current write controlvoltage 118 is re-applied to individual memory cells of the memory bank106 a to “flip” the logic state stored in the individual memory cellsfor the memory bank 106 a. For example, the adjustable voltage generatorcircuit 112 can generate a write control voltage 118 that is provided tothe control interface 110 of the memory IC 102. The controller interface110 of the memory IC 102 applies the write control voltage 118 to thememory cells of the memory bank 106 a. Assuming that the previous logicstate was “1”, the individual memory cells can receive the same writecontrol voltage 118 to “flip” the logic states from “1” to an expectedlogic state of “0”.

At step 360, during the second read cycle of the functional test, acurrent read control voltage 120 is applied to individual memory cellsof the memory bank 106 a to enable reading the actual logic statesstored in the memory cells of the memory bank 106 a. For example, theadjustable voltage generator circuit 112 can generate a read controlvoltage 120 that is provided to the control interface 110 of the memoryIC 102. In an embodiment, the current read control voltage is a fraction(e.g. 50%) of the current write control voltage 118. The controllerinterface 110 of the memory IC 102 applies the read control voltage 120to the memory cells of the memory bank 106 a. The read control voltage120 causes the memory cells of the memory bank 106 a to each generate asecond read current 122 that is forwarded to the functional tester 104by the control interface 110 for evaluation.

In step 362, the read current is measured for each memory cell of thememory bank 106 a for a second time. For example, the read currentmeasurement circuit 114 in the functional tester 104 measures the readcurrent 122 to generate a second read current measurement 124 for eachmemory cell in the memory bank 106 a. Generally, a relatively low readcurrent 122, indicates that a memory cell of the memory bank 106 a is ina high resistance state, which represents a logic “0” being stored inthe memory cell. Whereas, a relatively high read current 122, indicatesthat the memory cell of the memory bank 106 a is in low resistancestate, which represents a logic “1” being stored in the memory cell ofthe memory bank 106 a. The read current measurement circuit 114 providesa second read current measurement 124 for each memory cell in the memorybank 106 a to the controller 116, resulting in a second set of readcurrent measurements 124 that are received for evaluation. Given thenumber of memory cells in the memory bank 106 a-d under test, thisresults in a second distribution of read currents that represent a logic“0”, e.g. high resistance state assuming the memory bank 106 apreviously stored a logic “1” and was flipped to a logic “0” by thecurrent write control voltage 118.

After step 362, both the high resistance current distribution and thelow resistance current distribution for the memory bank 106 a have beenmeasured for the current write control voltage 118, as respectivelyillustrated by current distributions 204-1 and 206-1 shown in FIG. 2.Further, the number of error currents 208-1 are also apparent.Accordingly, at step 363, the error currents are determined for thecurrent write control voltage 118. For example, the controller 116 canexamine the current distribution 202-1 and determine the error currents208-1, wherein each error current 208-1 has a magnitude that falls inbetween the recognized low resistance current distribution 204-1 and thehigh resistance current distribution 206-1. Accordingly, the logicstates represented by error currents 208-1 are indeterminate, andtherefore determine the overall error rate for the memory bank 106 awhen using the current write control voltage 118.

At step 364, the error rate that is associated with the current writecontrol voltage 118 is determined. For example, the controller 116 candetermine the error rate as: the (number of error currents 208) dividedby (the total number of memory cells in the memory bank 106 a), which isthe same as the number of read currents 122 generated during a givenfunctional test at a particular write control voltage 118.

At step 366, it is determined whether the current write control voltageis at the end of the range of available write control voltages. Asdescribed above, the write control voltage 118 is to be stepped inpredetermined voltage increments, (e.g., 0.1v) over the range ofavailable write control voltages, resulting in n-number of write controlvoltages 118 and corresponding write/read functional tests. Accordingly,the controller 116 can determine at step 366 if the current writecontrol voltage 118 is last write control voltage in the range ofavailable write control voltages 118. If yes, then control flows to 370.If not, then control flows to step 368.

At step 368, the current write control voltage 118 is incremented by apredetermined amount. For example, the controller 116 can instruct theadjustable voltage generator circuit 112 to increment the write controlvoltage 118 by a predetermined voltage amount, (e.g., 0.1v). After step368, control flows back to step 352 so the write/read functional testcan be re-run using the incremented write control voltage 118.

At step 370, the n-number of error rates corresponding to the n-numberof write control voltages 118 are compared, and a preferred writecontrol voltage 118 is selected based on the comparison. For example,the controller 116 can compare the error rates measured during thefunctional write/read test and select the write control voltage 118 thatprovides the lowest error rate from the n-number of error ratesdetermined. For example, referring to FIG. 2, the current distributions202-3 illustrate the least number of error currents 208, and thereforehave the lowest error rate. Accordingly, the controller 116 can selectthe corresponding write control voltage 118-3 of the range of writecontrol voltages since the write control voltage 118-3 produced thelowest error rate. Further, the preferred read control voltage 120 canbe selected as a fraction (e.g. 50%) of the preferred write controlvoltage 118. After step 370, control flows back to step 306 in flowchart300 of FIG. 3A.

FIG. 4 further describes a memory bank 400 accordingly to an exemplaryembodiment of the disclosure. For example, the memory bank 400 can be aMRAM or RRAM memory bank, and representative of one of memory banks 106a-d that shown in FIG. 1.

Memory bank 400 includes a memory cell array 410 that includes a numbermemory cells 412, arranged in rows and columns as shown. The memory bank400 further includes address input buffer 402, an x-decoder 406, ay-decoder 404, an x-selector 408, an output buffer 414, a y-selector416, a fuse/anti-fuse circuit 418, a fuse/anti-fuse circuit 420, aninput buffer 422, and buffer logic 424.

During operation, the address input buffer 402 receives an address thatidentifies a memory cell 412 of the memory array 410 that is to beaccessed for a read or write operation. The x-decoder 406 and y-decoder404 decode the address to respectively determine the row and column ofthe identified memory cell 412. The x-selector circuit 408 enables therow of the identified memory cell 412 based on the output of thex-decoder 406, and the y-selector circuit 416 enables the column of theidentified memory cell 412 based on the output of the y-decoder 404, sothat the identified memory cell 412 can be accessed for read or write.The input buffer 422 receives any data to be written to the identifiedmemory cell 412, and the output buffer 414 temporarily stores any dataread from the identified memory cell 412. The buffer logic circuit 424controls the timing of the read and write operations by controlling theinput buffer 422 and the output buffer 414. The fuse/anti-fuse circuit418 applies the preferred write control voltage 118 and/or read controlvoltage 120 that was determined for the memory bank 400 during thefunctional test as described in steps 208-210 of flowchart 200 for theprospective read/write operation. As described above, it is contemplatedthat each memory bank 106,400 will have a preferred write controlvoltage 118 and read control voltage 120. Additionally, the presentdisclosure can be extended so that individual memory cells 412 or agroup of memory cells within the memory bank 400 can have an individualpreferred write control voltage and an individual read control voltageas provided by the fuse/anti-fuse 420.

FIGS. 5A and 5B illustrate adjustable voltage generator circuits 502 and504 that are representative of adjustable voltage generator circuit 112according to embodiments of the present disclosure.

In FIG. 5A, adjustable voltage generator circuit 502 includes a numberof zener diodes 506 a-n that are series-connected, a resistor Rs, and amultiplexer 510. The adjustable voltage generator circuit 502 receivesan input voltage Vin and generates a number of voltages Vout-1 throughVout-n, which are stepped incrementally based on the voltage drop causedby each zener diode 506. The multiplexer 510 receives the voltagesVout-1 through Vout-n and selects one as the output Vout, based on acontrol signal from the controller 116. The selected output voltage Voutcan be the write control voltage 118 or the read control voltage 120 forapplication to the memory bank 106 a-d under test, as described above inflowcharts 200 and 300. Accordingly, using this technique, the writecontrol voltage 118 and read control voltage 120 can be repeatedly andincrementally stepped during the functional test of the correspondingmemory bank 106 a-d as described above in flowcharts 300 and 350. FIG.5A illustrates a voltage range from 2.5-3.5 volts, with 0.1 voltincrements, but other ranges and/or incremental steps could be used aswill be understood by those skilled in the arts based on the discussionherein.

In FIG. 5B, adjustable voltage generator circuit 504 includes a numberof resistors 508 a-n that are series-connected, a resistor Rs, and themultiplexer 510. The adjustable voltage generator circuit 504 receivesan input voltage Vin and provides a number of output currents Iout-1through Iout-n, which are stepped incrementally based on the incrementalresistance provided by each resistor 508. The multiplexer 510 receivesthe output currents Iout-1 through Iout-n and selects one as the outputIout, based on a control signal 126 from the controller 116. Theselected output current Iout, when driven through a known impedance, canprovide the write control voltage 118 or the read control voltage 120for application to the memory bank 106 a-d under test, as describedabove in flowcharts 300 and 340.

FIGS. 6A and 6B illustrate anti-fuse circuits 602 and 604 that arerepresentative of fuse/anti-fuse circuits 108 according to embodimentsof the present disclosure.

In FIG. 6A, anti-fuse circuit 602 includes a number of zener diodes 606a-n that are series-connected, a resistor Rs, and a multiplexer 610,similar in configuration to the adjustable voltage generator circuit502. The anti-fuse circuit 602 further includes a number of anti-fuses612 a-n that are each series connected to a corresponding zener diode606. Each anti-fuse 612 can provide a low impedance electricalconnection across its terminals upon being activated (or “blown”) aswill be understood by those skilled in the relevant art. For example,each anti-fuse 612 can be implemented with a capacitor structure havingan oxide disposed between two metal plates, where the oxide is “blown”upon application of a high voltage 614 to electrically short the metalplates and thereby activate the anti-fuse 612. The high voltage 614 canapplied via the multiplexer 610.

During operation, the anti-fuse circuit 602 receives an input voltageVin and provides a number of output voltages Vout-1 through Vout-n,which are stepped incrementally based on the voltage drop caused by eachzener diode 606. One anti-fuse 612 is selected to be blown, based on thepreferred write control voltage 118 or preferred read voltage 120 thatis determined during the functional testing described above forflowcharts 200 and 300. The multiplexer 610 combines all outputs of theanti-fuses 612, but only one anti-fuse 612 is activated so as to pass acorresponding control voltage as described above. Therefore, only one ofVout-1 through Vout-n corresponding to the activated anti-fuse 612appears at Vout, which is applied to the corresponding memory bank 106a-d to provide the preferred write control voltage 118 or the preferredread control voltage 120 to the corresponding memory bank 106 a-d duringfield application use.

In FIG. 6B, anti-fuse circuit 604 includes a number of resistors 608 a-nthat are series-connected, a resistor Rs, and a multiplexer 610, similarin configuration to the adjustable voltage generator circuit 504. Theanti-fuse circuit 604 further includes a number of anti-fuses 612 a-nthat are each series connected to a corresponding resistor 608 a-n. Asdiscussed above, each anti-fuse 612 can provide a low impedanceelectrical connection across its terminals upon being activated (or“blown”) as will be understood by those skilled in the relevant art.Each anti-fuse 612 can be implemented with a capacitor structure havingan oxide disposed between two metal plates, where the oxide is “blown”upon application of a high voltage 614 to electrically short the metalplates and thereby activate the anti-fuse 612. The high voltage 614 canapplied via the multiplexer 610.

During operation, the anti-fuse circuit 604 receives an input voltageVin and provides a number of output currents Iout-1 through Iout-n,which are stepped incrementally based on the incremental resistanceprovided by resistors 608 a-n. One anti-fuse 612 is selected to beblown, based on the preferred write control voltage 118 or preferredread voltage 120 that is determined during the functional testingdescribed above for flowcharts 200 and 300. The multiplexer 610 combinesall outputs of the anti-fuses 612, but only one anti-fuse 612 isactivated so as to pass a current as described above. Therefore, onlyone of Iout-1 through Iout-n corresponding to the activated anti-fuse612 appears as Tout, and is applied to the corresponding memory bank106. Specifically, the selected output current Tout, when driven througha known impedance, can provide the preferred write control voltage 118or the preferred read control voltage 120 for application to the memorybank 106 a-d during field application use.

FIG. 7 illustrates fuse circuit 700 that is representative offuse/anti-fuse circuits 108 according to embodiments of the presentdisclosure. Fuse circuit 700 includes a number of resistors 702 a-n thatare series-connected, a number of fuses 704 a-n, a resistor Rs, and amultiplexer 706. Each fuse 704 provides a low impedance electricalconnection across its terminals in its nominal conducting state, but isdesigned to be “blown” (i.e. open-circuited) upon receiving a highthreshold current, as will be understood by those skilled in therelevant art. Each fuse 704 can be implemented with a thin conductorthat disintegrates upon receipt of a high current above the currentthreshold.

All the fuses 704 a-n with the exception of one are blown, based on thepreferred write control voltage 118 or preferred read control voltage120 that is determined during the functional testing described above forflowcharts 200 and 300. The multiplexer 706 combines all outputs of thefuses 704 a-n, but only one fuse 704 remains in a conducting state so asto pass a current. The fuses 704 can be blown by using high currents 706that are above the fuse threshold limit that are applied via themultiplexer 706. Therefore, only one of Iout-1 through Iout-n,corresponding to the surviving selected fuse 704 appears at Tout, and isapplied to the corresponding memory bank 106 a-d for application fielduse. Specifically, the selected output current Tout, when driven througha known impedance, can provide the preferred write control voltage 118or the preferred read control voltage 120 for application to the memorybank 106.

FIG. 8 illustrates an MRAM cell 800 according to embodiments of thepresent disclosure. For example, MRAM cell 800 can be representative ofone of the number of memory cells in the memory banks 106 a-d discussedherein, and/or memory cell 412 of memory cell array 410. MRAM cell 800is formed of a number of semiconductor layers in a stackedconfiguration, including metal layers 802 and 808, an adjustable magnetlayer 804, a permanent magnet layer 806, and a magnesium oxide layer 805that separates the adjustable magnet layer 804 from the a permanentmagnet layer 806. The MRAM cell 800 further includes a bit line terminal801, a source line terminal 812, and an access transistor 810.

During operation, the MRAM cell 800 is selected for write/read operationby activating the word line 811 so that the access transistor 810 willconduct (e.g. turn-on), as will be understood by those skilled in therelevant arts. The MRAM cell 800 stores a logic “1” when the magneticfield of the adjusted magnet layer 804 is aligned with that of thepermanent magnet layer 806, resulting in a relatively low resistance forcurrent flow between the bit line terminal 801 and the source lineterminal 812. The MRAM cell 800 stores a logic “0” when the magneticfield of the adjustable magnet layer 804 is mis-aligned (e.g. opposite)with that of the permanent magnet layer 806, resulting in a relativelyhigh resistance for current flow between the bit line terminal 801 andthe source line terminal 812. The logic state of the MRAM cell 800 canbe “flipped” by applying a high voltage (and corresponding current) tothe bit line terminal 801, causing the magnetic field of the adjustablemagnet layer 804 to change polarity as will be understood by thoseskilled in the arts. Accordingly, the write control voltage 118 and readcontrol voltage 120, discussed above, can be applied to the bit lineterminal 801 for write and read operations. Further, during the readoperation, the read current 122 can be measured from the source terminal812 to determine the current logic state stored in MRAM cell 800.

FIG. 9 illustrates an RRAM cell 900 according to embodiments of thepresent disclosure. For example, RRAM cell 900 can be representative ofone of the number of memory cells in the memory banks 106 a-d discussedherein, and/or memory cell 412 of memory cell array 410. RRAM cell 900is formed of a number of semiconductor layers in a stackedconfiguration, including metal layers 904 and 908, and a high-K oxidelayer 906. The RRAM cell 900 further includes a bit line terminal 902, asource line terminal 912, and an access transistor 910.

During operation, the RRAM cell 900 is selected for write/read operationby activating the word line 911 so that the access transistor 910 willconduct (e.g. turn-on), as will be understood by those skilled in therelevant arts. The RRAM cell 900 stores a logic “1” when the high Koxide layer 906 is in a low resistance state, caused by a high voltageor current creating low resistance “tunnels” through the high-K oxidelayer 906 as will be understood by those skilled in the arts. The RRAMcell 900 stores a logic “0” when the low resistance tunnels are brokenor not present. Similar to the MRAM, the write control voltage 118 andread control voltage 120, discussed above, can be applied to the bitline terminal 902 for write and read operations for the RRAM cell 900.Further, during the read operation, the read current 122 can be measuredfrom the source terminal 912 to determine the current logic state storedin RRAM cell 900.

Conclusion

The foregoing Detailed Description discloses a memory functional testerto perform a write/read functional test on a memory bank having a numberof memory cells over a number of write control voltages. The memoryfunctional tester includes an adjustable voltage generator circuit, aread current measurement circuit, and a controller. The adjustablevoltage generator circuit is configured to generate each write controlvoltage of the number of write control voltages to store first andsecond logic states in the number of memory cells during respectivefirst and second write cycles of the write/read functional test. Theread current measurement circuit is configured to measure first andsecond sets of read currents that define first and second sets of readcurrent distributions associated with the each write control voltage ofthe number of write control voltages. The first read currentdistribution represents the first logic state stored in the number ofmemory cells during the first write cycle, and the second read currentdistribution represents the second logic state stored in the number ofmemory cells during the second write cycle. The controller is configuredto determine a number of error currents that fall outside the first andsecond read current distributions associated with the each write controlvoltage of the number of write control voltages, determine an error rateassociated with the each write control voltage based on thecorresponding number of error currents associated with the each writecontrol voltage and a number of memory cells in the number of memorycells, compare the error rates associated with the number of writecontrol voltages with each other, and select a preferred write controlvoltage from the number of write control voltages based on thecomparison of the error rates.

The foregoing Detailed Description further discloses a method of testinga memory device having a number of memory banks. The method includesselecting a first memory bank of the number of memory banks, the firstmemory bank including a number of memory cells. A write/read functionaltest is performed on the first memory bank over a number of writecontrol voltages to determine a number of error rates, each error rateassociated with a corresponding write control voltage of the number ofwrite control voltages. Afterwhich, a preferred write control voltage isselected from the number of write control voltages based on the numberof error rates, wherein the preferred write control voltage isdesignated for use during subsequent write operations to the memory bankin an operational mode, and locking-in the preferred write controlvoltage for use in subsequent write operations to the first memory bank.

The foregoing Detailed Description further discloses a method ofmanufacturing a memory device having a number of memory banks. Themethod includes fabricating the memory device having the number ofmemory banks, and selecting a first memory bank of the number of memorybanks, the first memory bank including a number of memory cells. Awrite/read functional test is performed on the first memory bank over anumber of write control voltages to determine a number of error rates,each error rate associated with a corresponding write control voltage ofthe number of write control voltages. Afterwhich, a preferred writecontrol voltage is selected from the number of write control voltagesbased on the number of error rates, wherein the preferred write controlvoltage is designated for use during subsequent write operations to thememory bank in an operational mode, and locking-in the preferred writecontrol voltage for use in subsequent write operations to the firstmemory bank.

The foregoing Detailed Description referred to accompanying figures toillustrate exemplary embodiments consistent with the disclosure.References in the foregoing Detailed Description to “an exemplaryembodiment” indicates that the exemplary embodiment described caninclude a particular feature, structure, or characteristic, but everyexemplary embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same exemplary embodiment. Further, any feature,structure, or characteristic described in connection with an exemplaryembodiment can be included, independently or in any combination, withfeatures, structures, or characteristics of other exemplary embodimentswhether or not explicitly described.

The foregoing Detailed Description is not meant to limiting. Rather, thescope of the disclosure is defined only in accordance with the followingclaims and their equivalents. It is to be appreciated that the foregoingDetailed Description, and not the following Abstract section, isintended to be used to interpret the claims. The Abstract section canset forth one or more, but not all exemplary embodiments, of thedisclosure, and thus, is not intended to limit the disclosure and thefollowing claims and their equivalents in any way.

The exemplary embodiments described within foregoing DetailedDescription have been provided for illustrative purposes, and are notintended to be limiting. Other exemplary embodiments are possible, andmodifications can be made to the exemplary embodiments while remainingwithin the spirit and scope of the disclosure. The foregoing DetailedDescription has been described with the aid of functional buildingblocks illustrating the implementation of specified functions andrelationships thereof. The boundaries of these functional buildingblocks have been arbitrarily defined herein for the convenience of thedescription. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

Embodiments of the disclosure can be implemented in hardware, firmware,software, or any combination thereof. Embodiments of the disclosure canalso be implemented as instructions stored on a machine-readable medium,which can be read and executed by one or more processors. Amachine-readable medium can include any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputing circuitry). For example, a machine-readable medium can includenon-transitory machine-readable mediums such as read only memory (ROM);random access memory (RAM); magnetic disk storage media; optical storagemedia; flash memory devices; and others. As another example, themachine-readable medium can include transitory machine-readable mediumsuch as electrical, optical, acoustical, or other forms of propagatedsignals (e.g., carrier waves, infrared signals, digital signals, etc.).Further, firmware, software, routines, instructions can be describedherein as performing certain actions. However, it should be appreciatedthat such descriptions are merely for convenience and that such actionsin fact result from computing devices, processors, controllers, or otherdevices executing the firmware, software, routines, instructions, etc.

The foregoing Detailed Description fully revealed the general nature ofthe disclosure that others can, by applying knowledge of those skilledin relevant art(s), readily modify and/or adapt for various applicationssuch exemplary embodiments, without undue experimentation, withoutdeparting from the spirit and scope of the disclosure. Therefore, suchadaptations and modifications are intended to be within the meaning andnumber of equivalents of the exemplary embodiments based upon theteaching and guidance presented herein. It is to be understood that thephraseology or terminology herein is for the purpose of description andnot of limitation, such that the terminology or phraseology of thepresent specification is to be interpreted by those skilled in relevantart(s) in light of the teachings herein.

What is claimed is:
 1. A memory functional tester to perform a write/read functional test on a memory bank having a plurality of memory cells, the memory functional tester, comprising: an adjustable voltage generator circuit configured to generate each write control voltage of a plurality of write control voltages to store first and second logic states in the plurality of memory cells during respective first and second write cycles of the write/read functional test; a read current measurement circuit configured to: measure first and second sets of read currents that define first and second sets of read current distributions associated with the each write control voltage of the plurality of write control voltages, wherein the first read current distribution represents the first logic state stored in the plurality of memory cells during the first write cycle, and the second read current distribution represents the second logic state stored in the plurality of memory cells during the second write cycle; a controller configured to: determine a number of error currents that fall outside the first and second read current distributions associated with the each write control voltage of the plurality of write control voltages; determine an error rate associated with the each write control voltage based on the corresponding number of error currents associated with the each write control voltage and a number of memory cells in the plurality of memory cells, resulting in a plurality of error rates; compare a lowest error rate of the plurality of error rates with a predetermined threshold; and determine whether the memory bank has passed the write/read functional test based on the comparison of the lowest error rate with the predetermined threshold.
 2. The memory functional tester of claim 1, wherein the controller is further configured to: determine the memory bank has passed the write/read functional test based on the lowest error rate being less than the predetermined threshold; and mark the memory bank as pass so the memory bank can be subsequently used for a field application.
 3. The memory functional tester of claim 2, wherein the controller is further configured to: determine the memory bank has failed the write/read functional test based on the lowest error rate being greater than the predetermined threshold; and mark the memory bank as fail so the memory bank cannot be subsequently used for a field application.
 4. The memory functional tester of claim 1, wherein the controller is further configured to: compare the error rates associated with the plurality of write control voltages with each other; and select a preferred write control voltage from the plurality of write control voltages based on the comparison of error rates, wherein the preferred write control voltage is associated with the lowest error rate of the error rates.
 5. The memory functional tester of claim 4, wherein the controller is further configured to: determine a first read detection window to fit the first current distribution associated with the preferred write control voltage; and determine a second read detection window to fit the second current distribution associated with the preferred write control voltage, wherein a subsequent read current generated during the operational mode will be compared against with the first read detection window and the second read detection window to determine whether the subsequent read current represents the first logic state or the second logic state.
 6. The memory functional tester of claim 1, wherein for the each write control voltage, the adjustable voltage generator circuit is configured to: generate the each write control voltage a first time to store the first logic state in the plurality of memory cells; generate each read control voltage a first time to trigger read currents that define the first current distribution; generate the each write control voltage a second time to store the second logic state in the plurality of memory cells; and generate the each read control voltage a second time to trigger read currents that define the second current distribution.
 7. The memory functional tester of claim 1, wherein the adjustable voltage generator is configured to repeatedly increment the write control voltage by a predetermine amount to generate the plurality of write control voltages, and wherein each write control voltage is applied to the memory cells two successive times to store respective first and second logic states in the plurality of memory cells.
 8. The memory functional tester of claim 1, wherein the memory bank is one of Magnetic Random Access Memory (MRAM) memory bank or a Resistive Random Access Memory (RRAM) memory bank.
 9. A method, comprising: selecting a first memory bank of a plurality of memory banks, the first memory bank including a plurality of memory cells; performing a write/read functional test on the first memory bank over a plurality of write control voltages to determine a plurality of error rates, each error rate associated with a corresponding write control voltage of the plurality of write control voltages; comparing a lowest error rate of the plurality of error rates with a predetermined threshold; and determining whether the first memory bank has passed the write/read functional test based on the comparison of the lowest error rate with the predetermined threshold.
 10. The method of claim 9, further comprising: determining the first memory bank has passed the write/read functional test based on the lowest error rate being less than the predetermined threshold; and marking the first memory bank as pass so the first memory bank can be subsequently used for a field application.
 11. The method of claim 10, further comprising: determining the first memory bank has failed the write/read functional test based on the lowest error rate being greater than the predetermined threshold; and marking the first memory bank as fail so the first memory bank cannot be subsequently used for a field application.
 12. The method of claim 9, further comprising: selecting a preferred write control voltage from the plurality of write control voltages based on the plurality of error rates, wherein the preferred write control voltage is designated for use during subsequent write operations to the first memory bank in an operational mode.
 13. The method of claim 12, further comprising: locking-in the preferred write control voltage for use in subsequent write operations to the first memory bank.
 14. The method of claim 12, wherein the selecting comprises: comparing the error rates associated with the plurality of write control voltages with each other; and selecting the preferred write control voltage from the plurality of write control voltages based on the comparison of the error rates, wherein the preferred write control voltage is associated with the lowest error rate of the error rates.
 15. The method of claim 9, wherein the performing the write/read functional test on the first memory bank includes: applying each write control voltage to the plurality of memory cells to store first and second logic states in the plurality of memory cells during respective first and second write cycles; and measuring first and second sets of read currents associated with the each write control voltage of the plurality of write control voltages during respective first and second read cycles, wherein the first set of read currents includes a first read current distribution that represents the first logic state stored in the plurality of memory cells, and the second set of read currents includes a second read current distribution that represents the second logic state stored in the plurality of memory cells.
 16. The method of claim 15, further comprising: determining a number of error currents that fall outside the first and second read current distributions associated with each write control voltage of the plurality of write control voltages; and determining an error rate associated with the each write control voltage based on the corresponding number of error currents associated with the each write control voltage and a number of memory cells in the plurality of memory cells;
 17. A manufacturing method, comprising: fabricating a memory device having a plurality of memory banks; selecting a first memory bank of the plurality of memory banks, the first memory bank including a plurality of memory cells; performing a write/read functional test on the first memory bank over a plurality of write control voltages to determine a plurality of error rates, each error rate associated with a corresponding write control voltage of the plurality of write control voltages; comparing a lowest error rate of the plurality of error rates with a predetermined threshold; and determining whether the first memory bank has passed the write/read functional test based on the comparison of the lowest error rate with the predetermined threshold.
 18. The manufacturing method of claim 17, further comprising: determining the first memory bank has passed the write/read functional test based on the lowest error rate being less than the predetermined threshold; and marking the first memory bank as pass so the first memory bank can be subsequently used for a field application.
 19. The manufacturing method of claim 18, further comprising: determining the first memory bank has failed the write/read functional test based on the lowest error rate being greater than the predetermined threshold; and marking the first memory bank as fail so the first memory bank cannot be subsequently used for a field application.
 20. The manufacturing method of claim 17, further comprising: comparing the error rates associated with the plurality of write control voltages with each other; selecting a preferred write control voltage from the plurality of write control voltages based on the comparison of the error rates, wherein the preferred write control voltage is associated with the lowest error rate of the error rates; and locking-in the preferred write control voltage for use in subsequent write operations to the first memory bank, wherein the preferred write control voltage is designated for use during subsequent write operations to the first memory bank in an operational mode.
 21. The method of claim 9, further comprising: manufacturing the plurality of memory banks including the first memory bank. 